Behavioral Neuroscience Summers

Leech Shortening Sensory Stimulation of Shortening S&R Sensitization Learning Motor Efferents Neuromuscular Function Integration

Glu 5-HT ACh Leech figures Shortening Neurocircuitry end     Acronyms/Abbreviations

Sensitization - Leech
VII. Sensitization and Learning 			back to  S-R Sensitization

	A. Sensitization - simple or very complex learning?
	 
		1. Non-associative learning 
		
			a. change in behavior from experience with a single stimulus
			
				i. associative learning: relationship between 2 stimulus events
				
					(1) requires repetition 
			
		2. strong or noxious stimulus 
			
			a. persistently ñ response to a weaker (neutral) stimulus
				
			b. ñ responsiveness ñ with intensity of strong stimulus
			
			c. heterosynaptic facilitation

		3. Behavioral changes prevent habituation 
		
			a. Dehabitualization and sensitization have different mechanisms 
			
				i. similar behavioral consequences

		4. Sensitization may be important for more complex learning
		
			a. such as associative learning: 
			
				i. like classical conditioning, instrumental conditioning
			
				i. which may not require heterosynaptic facilitation
				
					(1) as with habituation 
			
			b. key feature of learning: 
			
				i. Discriminate learned and novel stimuli
	
		5. Elementary forms of learning: 
		     habituation, sensitization, classical conditioning
	
			
	B. Elementary learning - Habituation
		   
	   	1.  simplest form of implicit learning
		
			a. novel stimulus
			
				i. initial response = attention, orientation toward
			
			b. repeated exposure
			
				i. if neither harmful nor beneficial
				
			c. it is ignored = habituation
			
			d. habituation leads to decrease in the strength
			   of the synaptic connections
			     
				i. often between excitatory interneurons & motor neurons
			
		2. Habituation of shortening reflex
	
			a.  reduces S-cell excitability
		
			b.  low [5-HT] = decreased S-cell excitability
		
				i.  habituation mediated by reduced 5-HT release from R?  
			
		3. repeated stimulation results in habituation of reflexive motor action
		   
		   	a. progressively smaller excitatory synaptic 
			    potentials in interneurons/motor neurons
				
				i. due to a ß number of Glu vesicles released 
				   from sensory neurons
					   
				   	(1) no D in AMPA or NMDA receptors
					
			b. cellular mechanism for learning short-term memory
			
				i. monosynaptic plasticity
				
				ii. few repeated stimuli Þ minutes of habitiuation
			
			c. long-term memory
			
				i. many sessions Þ habituation for weeks
			
		4. Enhancement of synaptic inhibition could also contribute to habituation
		
			a. inhibitory interneurons
			
		5. Not all synapses are equally adaptable
	
		
	C. Elementary Learning: Sensitization
	
		1. Strong/harmful stimulus Þ more vigorous response
		
			a. differs in modality or intensity 
			
				i. from the (test) stimulus used to elicit a specific behavior

			b. usually triggers the same response 
			
				i. wherever the stimulus is applied 

			c. behavioral effects can be generalized 
			
				i. to subsequent behavioral responses 

		2. short term sensitization
		
			a. single stimulus Þ Ý excitatory sensory output lasting minutes
			
			b. heterosynaptic process 
			
				i. a result of both intrinsic and extrinsic processes
				
					(1) Intrinsic = modulation within stimulus 
					     response pathways of a specific behavior
					
						(a) Habituation = an intrinsic process
					
					(2) Extrinsic = involves neural components
					     outside the pathway of a specific behavior
					
						(a) Extrinsic sensitization broadly
						    alters responses to subsequent stimuli
						
						(b) many stimulus-response pathways
				
				ii. 5-HT Retzius neuron increases synaptic 
				    Glu output of S-cell
					
					(1) 5-HT-dependent sensitization is extrinsic in nature
					
			c. 5-HT Synapse on S cell
			
				i. action of 5-HT terminal has direct immediate effect 
				    on Glu S cell synapse
			
			d. 5-HT binds to 5-HT7  receptors
			
				i. 5-HT4/6/7-like receptors Þ Ý Gs È AC È cAMP È PKA
			
			e. Ý PKA Þ ß K+ channel current
			
				i. Ý Ca++ influx
			
			f. Ý PKA + PKC + Ca++ release Glu vesicles from pool
			
				i. enhance binding at active zones
				
			g. \Ý Glu stimulated postsynaptic current
 
		3. long-term sensitization (LTS is similar to LTP)
		
			a. 5 shocks Þ Ý excitatory current
			   lasting days to weeks
			   
			b. persistent 5-HT4/6/7-like activation of cAMP/PKA
			
				i. PKA activates CREB cAMP Response Element Binding Protein 
				
				ii. CREB binds CRE on DNA
				
				iii. new proteins and long-term memory   
				   
	D. Classical Conditioning	
	
		1. learning by associating one type of stimulus with another
		
			a. 5-HT also plays an important role in associative learning
		
		2. pairing an non-salient stimulus with salient stimulus
		
			a. salient stimulus directly causes a behavior
			
			b. non-salient stimulus associated with the salient stimulus
			   indirectly elicits behavior			

			   
	E. Behavioral circuitry is completed, enhanced or limited
	
		1. Via Interneuron neurochemical modification 
		
			a. often with neuromodulators or receptors
			
			b. or hormones
			
		2. sensitization may be required for a circuit to be complete
			
			a. priming
				
				i. gradient vs threshold 
	
			
		3. Output of behavioral circuitry is modified
		
			a. altered, repeated, delayed, sped up
			
				i. may require additional circuitry
				
					(1) interneurons
					
					(2) brain/ganglia
	
			b. temporal/spatial summation of sensory/interneuron
			   synaptic potentials
			   
			   	i. cause motor cells to discharge repeatedly
			
		4. Learning
		
			a. synaptic plasticity
			
				i. formation of new synapses
				
				ii. strengthening synaptic connections
				
					(1) unsilencing synapses
					
						(a) adding AMPA to 
						     NMDA-only synapses
						
					(2) increasing(a)/decreasing(b) synaptic current
					
						(a) LTP long-term potentiation 
						    or LTS long-term sensitization
						
						(b) LTD long-term depression